The present invention relates generally to microelectronic assemblies and more particularly relates to methods of mass producing semiconductor chip assemblies.
A semiconductor chip is generally connected to an external circuit element through contacts on the front face of the chip. For example in the tape automated bonding process (hereinafter referred to as the xe2x80x9cTABxe2x80x9d process), a dielectric sheet, such as a thin foil of polyimide, is provided with one or more bond windows and an array of metallic leads on one surface thereof. Each lead has one end integrally connected to terminals on the dielectric sheet and an opposite end extending outwardly from a central portion of the dielectric sheet so that the outermost ends of the leads project beyond the bond windows. The dielectric sheet is juxtaposed with the semiconductor chip so that the bond windows are aligned with the contacts on the chip and so that the outermost ends of the leads overlie the front face of the chip. The leads are then bonded to the contacts of the chip using bonding techniques such as ultrasonic or thermocompression bonding. After the bonding step, the terminals are connected to an external circuit element, such as a printed circuit board, which electrically interconnects the chip and the printed circuit board.
Commonly assigned U.S. Pat. No. 5,148,266, the disclosure of which is incorporated by reference herein, discloses a method of manufacturing semiconductor chip assemblies which are fabricated in a substantially continuous sheet or strip. A plurality of connection components are spaced lengthwise along a continuous tape, each connection component having terminals and flexible leads thereon. In one assembly method, semiconductor chips are connected to respective connection components on the tape and the assembled semiconductor chips are then carried downstream with the tape for further processing steps.
Commonly assigned U.S. Pat. No. 5,659,952, the disclosure of which is incorporated by reference herein, provides methods of fabricating a semiconductor chip assembly having a compliant interface. In preferred methods according to U.S. Pat. No. 5,659,952, a flexible, substantially inextensible dielectric film having a surface is provided and a plurality of compliant pads are attached to the first surface of the dielectric film, whereby any two adjacent compliant pads define a channel therebetween. Attaching the compliant pads to the dielectric film may be accomplished in a number of different ways. In one embodiment, a stencil mask having a plurality of holes extending therethrough is placed on top of the first surface of the dielectric film. The holes in the stencil mask are then filled with a curable liquid elastomer. Desirably, liquid elastomer has a thick enough consistency so that the mask may be removed before curing the elastomer. After the mask has been removed, the elastomer is at least partially cured using energy, such as heat or ultraviolet light. The holes in the mask are preferably filled with the liquid elastomer by screening the liquid elastomer across an exposed surface of the mask such that the elastomer is deposited into the holes of the mask. Thus, there is provided an assembly which includes an array or plurality of compliant pads defining channels therebetween, i.e. the channels run between adjacent compliant pads.
In further stages of the process disclosed in U.S. Pat. No. 5,659,952, the assembly including the array of compliant pads is assembled to a second support structure, such as a semiconductor chip having a front face with contacts. During the assembly step, the front contact bearing face of the chip is abutted against the array of compliant pads and the contacts are electrically connected to terminals on a second surface of the dielectric film remote from the chip. A compliant filler, such as a curable liquid elastomer, may then be injected into the channels between the semiconductor chip and the dielectric film and around the compliant pads while the chip and the dielectric film are held in place. The curable liquid elastomer may then be cured to form a substantially uniform, planar, compliant layer between the chip and the dielectric film.
However, further improvements in handling of the components during assembly processes, such as those described in U.S. Pat. No. 5,659,952, would be desirable.
In accordance with one aspect of the present invention, a method of making a microelectronic assembly includes the steps of providing a flexible tape having first and second surfaces and including a plurality or array of connection components in a central region thereof. The flexible tape typically includes a dielectric film, such as a polyimide, whereby each connection component includes a part of the dielectric film having electrically conductive parts. The flexible tape includes one or more border regions which surround the central region bearing the plurality of connection components. The electrically conductive parts typically comprise an array or plurality of conductive terminals accessible at one surface of the dielectric film and may also include flexible leads integrally connected to the terminals. Each connection component also includes an attachment region for receiving a resilient element, such as an array or plurality of compliant pads, as will be discussed in more detail below. The attachment region of each connection component are preferably provided at the first surface of the flexible tape with the terminals preferably accessible at the second surface of the flexible tape.
A carrier frame, having a top surface and a bottom surface, is typically provided for processing the flexible tape. The carrier frame is desirably between approximately 250-400 microns thick and comprises a rigid material, such as a metal or plastic. The carrier frame has one or more inner edges which define a slot extending between the top and bottom surfaces thereof. In certain preferred embodiments the slot is elongated and the carrier frame includes a cut-out region which is contiguous with one end of the slot, whereby the width of the cut-out region is greater than the width of the slot.
In further stages of preferred assembly methods, the flexible tape is placed on a top surface of a work holder and the carrier frame is placed over the flexible tape so that the second surface of the flexible tape (i.e., the terminal side) is in contact with the surface of the carrier frame. The width of the flexible tape is greater than the width of the slot. Therefore, when the flexible tape overlies the carrier frame and is substantially parallel to and in contact therewith, the side border regions of the tape preferably extend beyond the one or more of the inner edges of the carrier frame. However, the width of the cut-out region is greater than the width of the flexible tape so that portions of the flexible tape overlying the cut-out region are typically bounded by the cut-out region.
In further stages of the process, one end of the flexible tape is secured to the carrier frame and is preferably hingedly or pivotally secured thereto. In certain preferred embodiments, the portion of the flexible tape extending into the cut-out region is pivotally secured to the carrier frame by affixing flexible strips to the border regions of the flexible tape overlying the cut-out region. The flexible strips may be removed and desirably include an adhesive thereon which secures to the border region of the tape. In certain preferred embodiments, each flexible adhesive strip has a first end connected to the border regions of the flexible tape overlying or extending into the cut-out region and a second end connected to the carrier frame.
In the next stage of the process, during a first processing operation, a resilient element is provided over the attachment region of each connection component. The resilient element may be provided by forming a plurality or array of compliant pads on each attachment region, such as by using the methods described in the aforementioned U.S. Pat. No. 5,659,952. The compliant pads are preferably formed by stenciling or screen printing an array of individual pads on each attachment region, whereby the array includes channels running between adjacent pads. During the step of providing the resilient elements on the attachment regions of the connection components, the flexible tape is maintained in a substantially stationary position over the top surface of the carrier frame by supporting the underside of the flexible tape. This underside support may be provided using a supporting element, such as a support plate, which passes through the slot in the carrier frame and engages the second surface of the flexible tape to prevent the flexible tape from flexing or moving downward during the providing a resilient element step. Although the present invention is not limited by any particular theory of operation, it is believed that the formation of properly aligned and shaped resilient elements is greatly facilitated when the bottom of the stencil is in direct contact with the first surface of the flexible tape. This direct contact is not possible when the carrier frame lies between the bottom of the stencil and the flexible tape. For example, if the flexible tape was under the carrier frame, rather than overlying the top of the carrier frame, then it would be impractical to place the bottom of the stencil directly in contact with the first surface of the flexible tape because the stencil would be spaced from the first surface of the tape by the carrier frame. As mentioned previously, this could result in the formation of misaligned and improperly shaped compliant pads on the various attachment regions of the connection components. The compliant pads may then be at least partially cured, such as by using heat or ultraviolet light. In other embodiments, the compliant pads may not be cured until after die attach, as will be discussed in more detail below.
After the compliant pads are formed on the attachment regions of the flexible tape, the tape is preferably passed through the slot in the carrier frame by disengaging the borders of the tape from the top surface of the carrier frame, passing the tape through the slot and re-engaging the borders of the tape with the bottom surface of the carrier frame. In accordance with one preferred embodiment of the present invention, the flexible tape initially overlies the top surface of the carrier frame with the second surface of the tape in contact with the carrier frame and with one end of the tape pivotally connected to the carrier frame. Preferably, the portion of the flexible tape which is pivotally connected to the carrier frame is that portion which overlies the cut-out region of the slot. As the flexible tape is held in a substantially stationary position, one end of the carrier frame is rotated about the pivotally secured portion of the flexible tape. The carrier frame is rotated upward between approximately 15-60xc2x0 and preferably between approximately 25-35xc2x0 from the initial or first position toward a second position. As the carrier frame moves toward the second position, the second surface of the flexible tape disengages from the top surface of the carrier frame and passes through the slot. After the flexible tape has disengaged from the top surface and passed through the slot, the carrier frame is then rotated downward approximately 25-35xc2x0 from the second position back toward the first position, and preferably all the way back to the first position, so that the bottom surface of the carrier frame engages or is in contact with the first surface of the flexible tape. During the passing the flexible tape step, the flexible nature of the tape permits the border regions of the flexible tape to move or flex toward one another so that the tape may pass freely through the slot. If the side borders of the tape were not capable of flexing inwardly, then the tape could not readily pass through the slot because, as mentioned above, the width of the flexible tape is greater than the width of the slot. After the passing step, the first surface of the flexible tape is in contact with the bottom surface of the carrier frame and the slot in the carrier frame overlies and is in substantial alignment with the central region of the flexible tape so that the slot overlies the connection components, which in turn are accessible through the slot. In this stage of the assembly process, the resilient elements on the respective connection components face upwardly in the slot.
In the next stage of the assembly process, during a second processing operation, microelectronic elements, such as semiconductor chips having contacts on a front face thereof, are assembled with the resilient elements of each connection component to provide microelectronic assemblies. The chips are preferably assembled to the resilient elements by abutting the chips against the resilient elements. During the assembling step, the flexible tape is maintained in a substantially stationary position, such as by using a supporting plate, to prevent the tape from flexing or moving. Supporting the underside of the flexible tape at this stage is important because if the tape flexed downward during assembly of the microelectronic elements, then the elements could possibly be misaligned over the connection components, thereby resulting in the production of defective chip assemblies.
After the die attach or assembly step, the microelectronic elements and the connection components are electrically interconnected, such as by bonding the flexible leads of the connection components to the contacts on the microelectronic elements, or by other known connecting processes. A curable liquid encapsulant, such as silicone elastomer, may then be provided between the microelectronic elements and the connection components. The curable liquid encapsulant preferably flows between the microelectronic element and the connection component, through the channels between the compliant pads and around the flexible leads. In certain embodiments, one or more coverlays may be used to prevent the liquid encapsulant from flowing into contact with the second surface of the connection components and/or surfaces of the chips, such as the back surfaces of the chips. The curable liquid encapsulant may then be cured using heat or ultraviolet light to provide a compliant interface for each microelectronic assembly. The cured encapsulant also protects the final assembly, including the flexible leads, from contamination. The assemblies may then be severed from the flexible tape by cutting around the perimeter of the assembly to provide individual microelectronic assemblies, or groups of two or more assemblies, capable of being interconnected with external circuit elements such as printed circuit boards.
In further preferred embodiments, the slot in the carrier frame is an elongated slot having first and second ends and first and second sides, and the cut-out region at one end of the slot is contiguous with either the first or second end of the slot. The flexible tape preferably includes an elongated strip having an array of connection components, such as an array of connection components aligned in a 3xc3x9710 matrix. The sides of the flexible tape are preferably bound by border regions on opposite sides thereof so that when the flexible tape overlies the carrier frame, and is substantially parallel thereto, the side border regions of the flexible tape extend beyond the sides of the elongated slot. However, the side border regions of the flexible tape extending into the cut-out region do not extend beyond the sides of the cut-out region because the width of the cut-out region is greater than the width of the flexible tape. Thus, during the pivotally securing steps described above, when the flexible strips are applied, to pivotally secure the tape to the carrier frame, the one or more strips are preferably attached to the portions of the side border regions extending into the cut-out region. Without the oversized cut-out region, it would be difficult to pass or transfer the flexible tape from one surface of the carrier frame to the opposite surface of the carrier frame while the flexible tape remained secured to the carrier frame.
Thus, by using a carrier frame having an elongated slot and a cut-out region as described above, it is possible to easily handle and manipulate the connection components and microelectronic assemblies through all of the assembly steps to the final severing operation. The flexible tape can be easily moved between the top and bottom surfaces of the carrier frame to provide unfettered access to the first and second surfaces of the flexible tape as required during different stages of the assembly process. The ability to quickly and easily transfer the tape from the top of the carrier frame to the bottom of the carrier frame simplifies the assembly process and minimizes handling of the flexible tape and the microelectronic elements. Moreover, the carrier frames can be readily manipulated (i.e. inverted) during the various assembly steps and are inexpensive to manufacture.
In another embodiment of the present invention, the carrier frame is substantially similar to that described above; however, the elongated slot includes teeth or projections which extend from opposite sides of the slot and toward the center of the slot. The side borders of the flexible tape contact the teeth to maintain the tape on the top or bottom surface of the carrier frame, as may be required during various stages of the assembly process. The flexible tape is pivotally connected to the carrier frame so that the tape can pivot between engagement with the top and bottom surfaces of the carrier frame as described above.
In another embodiment, an apparatus for processing flexible tape pivotally secured to a carrier frame which has a top surface and a bottom surface and a slot extending therebetween includes a base having a top surface and a bottom surface and including an aperture extending therebetween and a platform having a top surface and being sized to fit within the aperture in the base. The base is pivotally secured to one end of the platform and is movable between a first position wherein the top surface of the base is substantially parallel to the top surface of the platform and a second position wherein the top surface of the base has been pivoted between approximately 15-60xc2x0, and preferably between 25-35xc2x0 above the top surface of the platform. The carrier frame includes alignment apertures and the top surface of the base includes alignment posts so that the carrier frame may be aligned over the top surface of the base, whereby the slot in the carrier frame is aligned over the aperture in the base and the platform. The apparatus also includes a clamp which has a perimeter which is sized to pass through the slot in the carrier frame and the aperture in the platform so that the clamp may secure the flexible tape to the top surface of the platform as the platform moves between the first and second positions. The platform includes a securing element at the top surface thereof, such as a plurality of vacuum holes. The clamp and the vacuum holes cooperatively secure the flexible tape to the top surface of the platform as the base pivots between the first and second positions. The top surface of the platform is approximately 500-700 microns higher than the top surface of the base when the base is in the first position.
The foregoing and other objects and advantages of the present invention will be better understood from the following detailed description of preferred embodiments taken together with the attached figures.